\TYPO3\CMS\Extbase\Utility\ExtensionUtility::registerPlugin(
    $_EXTKEY,
    'pluginname',
    'Plugin description'
);

// Include flex forms
$pluginSignature = str_replace('_', '', $_EXTKEY) . '_' .    'pluginname'; // from registerPlugin(...)
$TCA['tt_content']['types']['list']['subtypes_addlist']   [$pluginSignature] = 'pi_flexform';
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPiFlexFormValue(
    $pluginSignature,
    'FILE:EXT:' . $_EXTKEY . '/Configuration/FlexForms/flexform_pluginname.xml'
);

--------------------------------------------

<T3DataStructure>
	<meta>
		<langDisable>1</langDisable>
	</meta>
	<sheets>
		<sDEF>
			<ROOT>
				<TCEforms>
				<sheetTitle>Price Settings</sheetTitle>
				</TCEforms>
				<type>array</type>
				<el>
					<settings.chars_per_row>
					<TCEforms>
					<label>Character Per Row</label>
					<config>
					<type>input</type>
					<default>55</default>
					</config>
					</TCEforms>
					</settings.chars_per_row>
				</el>
			</ROOT>
		</sDEF>
 	</sheets>
</T3DataStructure>